Debt and Financing Arrangements - Letter of Credit and Revolving Credit Facilities (Details) - USD ($)	Dec. 31, 2024	Dec. 31, 2023
Line of Credit Facility [Line Items]
Advances from Federal Home Loan Banks	$ 0	$ 0
Credit Facility
Line of Credit Facility [Line Items]
Optional increased capacity	1,500,000,000
Credit Facility | Asset Pledged as Collateral | Letter of Credit
Line of Credit Facility [Line Items]
Investments securing LOCs	400,000,000
Secured letter of credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	425,000,000
Outstanding borrowings	275,000,000
Remaining capacity	150,000,000
Unsecured revolving loan and letter of credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	500,000,000
Outstanding borrowings	0	$ 0
Remaining capacity	500,000,000
Wholly-owned subsidiaries | Arch Re Bermuda | Unsecured Letter of Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	175,000,000
Remaining capacity	69,000,000
Letters of credit outstanding, amount	106,000,000
Wholly-owned subsidiaries | Arch Re Bermuda | Lloyds Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	700,000,000
Letters of credit outstanding, amount	700,000,000
Wholly-owned subsidiaries | Arch Re Bermuda | LOC Facilities
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 55,000,000